Alternating current vacuum circuit interrupter



J. D. COBINE Dec. 19, 1961 ALTERNATING CURRENT VACUUM CIRCUIT INTERRUPTEFiled Oct. 29, 1959 k QWQQQ U dd??? SD00 In z /4 23? ls Attorney inventor-ztaneously from that value to zero.

United States Patent Ufhce 3,014,110 Patented Dec. 19, 1961 The presentinvention relates generally to vacuum-type electric circuit interruptersuniquely adapted for use in inductive circuits wherein thecurrentinterrupted is of low magnitude.

The phenomena of electric current interruption in atmospheric or higherpressures, on one hand, and in high vacuum, on the other hand, areextremely complex and basically different in nature. So also, are theproblems which must be overcome in producing operative devices toperform either function. Thus, for example, one great problem innon-vacuum circuit interrupters is the deterioration of the contactsthereof due to the formation of oxides of the contact metaland otherchemical compounds due to the presence of atmospheric air, a protectivegaseous atmosphere, or an oil bath. Any compound so formed is readilyseparated from the interrupter contacts. Such a continuous formation andremoval of non-metallic compounds rapidly causes the interruptercontacts to erode and deteriorate.

In the vacuum interrupter, on the other hand, the complete absence ofgases or liquids in contact with the interrupter contacts virtuallyprecludes the formation of contact-eroding oxides and compounds.

Other problems, however, not generally considered as significant in thenon-vacuum circuit interrupter, are of prime importance in theconstruction of vacuum circuit interrupters. Vacuum interruptersdesigned to interrupt alternating currents operate substantially asfollows:

When an arc is struck between the electrodes of a vacuum interrupter, asfor example, by the opening of a pair of switch contacts maintained in avacuum of 10- mm. of mercury pressure or less, the arc vaporizes some ofthe electrode material. If the contacts have been properly processed toremove therefrom all occluded and sorbed gases, only ionized metallicvapor is present in the arc. The are continues until the alternatingcurrent sought to be interrupted falls to a natural zero value, asnormally occurs once each alternation. When this occurs, the arc isextinguished, and the ionized metallic particles rapidly diffuse to thecold vacuum chamber walls where they are cooled, de-ionized, andcondensed. When the interrupted current seeks to rise again, the arccannot be reestablished because of the high dielectric strength of thevacuum separating the interrupter contacts. Thus, in a vacuum circuitinterrupter, the current is usually completely extinguished at the firstnaturally occurring instantaneous zero value of current.

The foregoing occurs satisfactorily in vacuum circuit interruptersoperating to interrupt alternating currents of high current ratings,namely of 500 amperes or greater. For lower values of alternatingcurrent, the phenomenon generally denominated as chopping occurs. Atthese low alternating current values, the arc, rather than persistinguntil the first natural current zero, is abruptly extinguished atsomelow current value and falls instan- The value of current at whichinstantaneous arc extinction occurs is denominated the chopping currentof the device. Typical chopping currents in prior art vacuuminterrupters may be from to 40 amperes. While chopping in vacuuminterrupters associated with capacitive circuits may be tolerated, it isunacceptable with inductive loads because of voltage surges inducedtherein by the high rate of change of current with time (di/a't) when aninterrupter chops. This may be seen from the relationship:

V=the surge voltage induced by chopping I =the chopping current L=theequivalent inductance of the circuit C=the equivalent capacitance of thecircuit,

and the fact that the For most industrial inductive circuit loads, it isnecessary to reduce the chopping current level of a vacuum circuitinterrupter to a value of below 4 amperes and,-

in most instances to a value of about 2 amperes.

A second important electrical characteristic of a vacuum circuitinterrupter is the voltage recovery char acteristic. This characteristicreflects the ability of the interrupter to withstand the rapidlyincreasing instantaneous electrode-to-electrode voltage following arcinterruption. This instantaneous electrode-to-elecrode voltage resultsfrom the combined efiects of the applied system alternating currentvoltage, and anytransient voltages associated with arc interruption,lightning, etc. In any given circuit interrupter application it isessential that the dielectric strength of the electrode-to-electrodespace shall at all times exceed the actual voltage appearing across theelectrodes. Otherwise the arc may restrike and the basic purpose of thecircuit interrupter is defeated.

In accordance with the present invention, the properties of low workfunction materials are advantageously utilized to minimize vacuumswitch. Materials of this class include, among others, thorium, caesium,lanthanum, rubidium, barium, and especially thoriated tungsten. All arecharacterized by the ability to produce electrons in the presence of anelectric field, and particularly under the influence of the strongfields and high temperatures at the points of arc contact in a vacuumswitch. The low work function material serves this purpose even thoughthe quantity ofin the arcing region is otherwise incapable metallicvapor of maintaining the arc. The low work function material ishomogeneously distributed throughout the material of at least one of theelectrodes, and preferably in both. When the arc is drawn, the low workfunction material provides a copious supply of electrons at the timechopping would otherwise take place, thereby providing a continuedcurrent-conducting path and prolonging the period during which the arcis maintained and thus providing a lower current level at interruption.Insofar as an interruption, or a series of interruptions, may otherwisetend to exhaust the availability of the low work function material atthe arcing face of the electrode, diffusion of such material from themore interior portions of the electrode serves to replenish the materialavailable at the point of arcing and thereby provide a long effectiveelectrode life.

An important aspect of the vacuum circuit interrupter of the presentinvention lies in the fact that the electrode is not, in the absence ofthe low work function material, capable of providing a low choppingcurrent. bly the electrode is formed of a refractory matrix impregnatedwith a low resistance material such as copper. An electrode so formed ischaracterized by a high degree of resistance to the corroding effects ofthe are, by low electrical resistance, and by other desirable characthechopping current value of a Prefera-- teristics. It is, however,relatively unsatisfactory in terms of chopping current, giving a valueof perhaps 6 amperes (which cannot be tolerated in some circuits). Thischopping current is due to the inadequate quantity of ionized electrodematerial available in the arc path when the current falls to about 6amperes. This same are starvation in terms of ionized electrodematerial, however, is desirable in providing a favorable voltagerecovery characteristic following interruption. While the low Workfunction material provided in accordance with the present inventionserves to maintain the are to a more satisfactory chopping currentvalue, it does not unduly increase the quantity or" vaporized electrodematerial at the time of interruption. Since the available electronsquickly disperse and disappear upon interruption of the arc, therecovery voltage of the vacuum circuit interrupter of the presentinvention is of the same order as the same circuit interrupter in theabsence of the low work function material.

In accordance with the preferred embodiment of tr e present invention,at least one electrode of the interrupter is formed from a body oftungsten, serving both as refractory material and as an activating agentfor the low work function material. The low work function material inthis preferred embodiment is thorium, has the unusual characteristic ofcoacting with tungsten to provide an even lower work function than thatof elemental thorium.

It is therefore a general object of the present invention to provide animproved vacuum circuit interrupter in which chopping current isminimized by the action of a low work function material.

An additional object of the present invention is to provide an improvedvacuum circuit interrupter utilizing a low work function material inconjunction with electrode materials that otherwise produce substantialchopping current, to provide both small chopping current and a rapidvoltage recovery.

Still another and more specific object of the present invention is toprovide an improved vacuum circuit interrupter in which at least oneelectrode is composed of a refractory material having a high degree ofresistance to the eroding action of the arc and coacting with a low workfunction material to activate the same in a manner providing anespecially favorable work function value and correspondingly lowchopping current.

It is still another object of the present invention to provide animproved alternating current vacuum circuit interrupter characterized bylow chopping current, a high recovery voltage rate, ease of manufacture,long life, and is otherwise constructed and arranged to provide a unithaving a maximum degree of commercial utility.

The novel features believed characteristic of the present invention areset forth with particularity in the appended claims. The inventionitself, however, together with further objects and advantages thereof,may best be understood by reference to the following description takenin connection with the attached drawing in which:

FIG. 1 is a representative view of a vacuum-type circuit interrupterconstructed in accord with the present invention and,

FIG. 2 is a graphical representation of a sinusoidual alternatingcurrent illustrating the efiect of the chopping phenomenon.

In FIG. 1 an interrupter chamber 10 comprises a wall member 11 which maybe cylindrical in shape and is constructed of a suitable insulatingmaterial, having at the ends thereof a pair of metallic end members 12and 13 closing the volume therein to form an interrupter chamber.Suitable seals 14 are provided between casing 11 and end members 12 and13 to render the interrupter chamber vacuum-tight.

Located within chamber 10 are a pair of separable contacts orarc-electrodes 15 and 16 which are shown in their closed circuit orengaged position. Upper contact 15 is which a stationary contactsuitably attached electrically and mechanically to a conducting rod 17which, at its upper end, is united electrically and mechanically withend member 12. Lower contact 16, mounted upon and electrically unitedwith a suitable conducting rod 18, is movable and is connected throughbellows 26 or an equivalent vacuum tight member permitting reciprocatingmotion. Terminal mount 18 projects through a suitable orifice in endmember 13, and suitable actuating means may be connected thereto toimpart reciprocating motion to rod 18 to cause contact 16 to enter intoengagement with, and be removed out of engagement with contact 15. Forcertain types of interrupters as for example, vacuum fuses and lightningarresters, electrode 16 need not be movable but may be spaced apart fromelectrode 15 a suitable distance. The electrical circuit which is soughtto be interrupted by the interrupter device may be completed by makingsuitable connections to contact 21, electrically and mechanicallymounted upon end member 12 and terminal 22, electrically andmechanically mounted upon rod 18. A suitable insulator shield, such asmetallic cylindrical member 23, capped with an arc-preventing ferrule 24is interposed between electrodes 15-16 and insulator 11 to prevent thelatter from becoming coated with metal.

The volume within interrupter chamber 10 is suitably evacuated throughan exhaust tubulation (not shown) during the final assembly thereof. Forproper operation of the interrupter as a vacuum-type interrupter ofalternating currents, the pressure within chamber 10 must be maintainedat a pressure at least as low as 10" mm. of mercury, but is preferablymaintained within the range of 10 to 10 mm. of mercury. The foregoingrequirement is essential for the operation of the devices as vacuuminterrupters of alternating currents. This requirement is necessarybecause, in order that the currentcarrying arc struck between electrodes1.5 and 16 be extinguished at the first occuring current zero value,there must be susbtantially no ionizable gas present within chamber 10.The occurrence of ionization may be substantially prevented if thepossible breakdown paths between electrodes 15 and 1.6, or theirrespective supports, are small with respect to the mean free path of anelectron within the atmosphere obtained with the device. This mean freepath is designated as the statistical distance which an electron maytravel without colliding with a gas molecule at a given pressure. Theseconditions may be established in operative circuit interrupters onlywhen the pressure within interrupter chamber is below lO mm. of mercuryand preferably below 10" mm. of

mercury.

In FIG. 2 of the drawing there is shown, in graphical form, anillustration of the chopping phenomenon. In FIG. 2, the instantaneousvalue of a sinusoidal alternating current which is sought to beinterrupted by a vacuum circuit interrupter is plotted for one-halfcycle. As the current depicted by curve A rises from instantaneous valueof zero, the contacts, as for example, contacts 15 and 16 in FIG. 1, areseparated at point B causing an arc discharge to be establishedtherebetween. Initially, this are discharge is largely sustained by themetal evaporated from contacts 15 and 16 by the heat generated at thecontact surfaces by the arc. The terminal points of the are are knownrespectively as cathode and anode spots. As a matter .of practice, mostof this evaporation occurs at the cathode, or negatively maintainedelectrode. It is, however, difficult to predetermine which electrode isnegative at a particular instance when an alternating current circuit isinterrupted. In FIG. 2 the value of current in the arc follows itsnatural course along the sinusoid of curve A and, for high current arcs(those above 500 amperes) follows the dotted line until a zero value isreached. At this instant the are is extinguished and the energizedmetallic ions between the electrodes rapidly diffuse to the cold wallsof members 12, 13 and 1 zero value but, rather,

23 where they are cooled and deionized. The are remains extinguishedbecause, when a high voltage builds up between contacts and 16, the areis not reestablished, due to the high dielectric strength of the vacuumseparating the contacts.

In the operation of vacuum circuit interrupters wherein the currentsought to be interrupted is of a relatively low value, (below 500amperes) the instantaneous current value does not follow the dotted linecontinuously to a at some low current value, denominated by I andoccurring at a time 0, the arc is abruptly and prematurely extinguished.This results in an instantaneous change of current from a value of I tozero. The value I is referred to herein as the chopping current valuefor a particular device. As will be readily appreciated, this almostinstantaneous change of current from I to zero results in a high rate ofchange of current with time (di/dt) and results in the production ofextremely high surge voltages and inductive loads which may be connectedthereto. These surges may cause the breakdown of insulation and aregenerally highly injurious to electrical equipment.

In accordance with the copending application of T. H. Lee and J. D.Cobine, Serial Number 750,784, filed June 24, 1958, and assigned to thesame assignee as the present invention, the disclosure of which isincorporated herein by reference, the chopping current problem isovercome by an appropriate selection of electrode materials to providevapor pressure sufficient to counterbalance vapor starvation responsiblefor such large chopping current values. This general technique cansatisfactorily overcome the chopping current problem in many vacuumswitch applications. However, in those instances where the choppingcurrent must be reduced to a rather small value this technique also maygive rise to a somewhat prolonged availability of vaporized electrodematerial, and hence 'a somewhat unfavorable recovery voltagecharacteristic.

In accordance with the present invention at least one, and preferablyboth, of the electrode contacts 15 and '16 includes a material having alow work function in conjunction with a major proportion of materialotherwise having a relatively unfavorable chopping current. Suchmaterials include thorium, caesium, lanthanum, rubidium, and barium.Thoriated tungsten is an especially preferred material for the reasonsdescribed hereafter. These and other materials with work functions lessthan 3.5 electron volts or below may be used. In the presence of theelectric field and temperature conditions existing at the point of arccontact-even at a low arc current-the low work function materials supplyelectrons in copious quantities. Their action is quite unlikethe currentconduction resulting from vaporization of the contacts 15 and 16 such aswhen large arc currents are being carried. In such vaporization thecontact materials assume gaseous form and then disassociate intoconducting ions and other conduction carriers. In contrast, the low workfunction material does not necessarily vaporize to provide conductioncarriers but instead emits electrons that pass into the vacuum space toconduct the current.

In accord with my invention I find that a range of from 2 to 15% byweight of low work junction material is sufiicient to achieve thedesired results. If less than 2% is used, there is insufficientthermionic emission to sustain the are. If greater than 15% is used themechanical strength of the arc-electrodes is adversely affected andexcessive eroding results.

By way of a specific and preferred example of the construction of thecontacts 15 and 16 the contacts may be composed of a refractory matrixof about 7% thorium, tantalum hydride binder, and the balance tungsten.The matrix may be made by homogeneously mixingthese materials, pressingthe same to the desired contact shape, and then heating to about 1900 C.in hydrogen for one hour. The contact matrix thus formed is thenimpregnated with metallic copper, preferably by heating the same in avacuum to temperature sufficient to melt copper and permit the same toimpregnate itself into the pores of the matrix. Tests have demonstratedthat the chopping current of a vacuum switch utilizing a pair ofcontacts formed in this fashion is of the order of 2.5 amperes. Thisvalue should be compared with the value of about 6 amperes for thecopper substituent of the electrodes and the very much greater value forthe tungsten substituent.

In the above embodiment ofthe present invention the tungsten serves as arefractory material. That is, it withstands a high temperature beforedisintegrating and hence resists the eroding action of the are. It alsoserves a quite different function in that it coacts with the thorium toprovide an even lower work function than the low work function ofelemental thorium. This action is thought to arise from a very thinsurface layer of thorium that is positive in relation to the tungstenbody of the electrode. Whatever the reason for this action, thethorium-tungsten electrode is characterized by a more favorable workfunction than elemental thorium and by the refractory characteristics ofthe tungsten.

A highly desirable feature of the present invention is that it enablesthe electrodes to be designed upon the basis of characteristics otherthan chopping current value. Thus, refractory materials such astungsten, molybdenum, tantalum, and the like may be utilized for theirresistance to the eroding action of the arc, and low resistance lowmelting point impregnating materials such as silver, copper, etc., maybe employed to provide a low electrical resistance. And, since thesematerials are not relied upon primarily to provide conduction carriersat the moment of arc interruption, it is unnecessary to utilize the veryhigh vapor pressure materials that might otherwise be necessary toreduce the chopping current. Since very high vapor pressure electrodematerials are not required it is unnecessary to sacrifice a favorablevoltage recovery characteristic in the interest of attaining a lowchopping current value. It will be appreciated, however, that in someinstances it may be desirable to utilize these materials in conjunctionwith the low work function materials of the present invention.

While I have shown and described only certain constructions within thepresent invention it will, of course, be understood that variousmodifications and alternative constructions may bemade without departingfrom the spirit and scope thereof. I therefore intend by the appendedclaims to cover all such modifications and alternative constructions asfall within the true spirit and scope thereof.

In the appended claims, the expression refractory material is used todesignate materials such as tungsten, with melting points in excess of3000" C. and the expression low work function material is used todesignate materials such as thorium with work functions of less than 3.5electron volts.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a pressure lower than 10- mm. ofmercury; a pair of electrical terminals adapted for connection in analternating current circuit; a pair of electrodes located within saidenvelope, connected in circuit between said terminals and disposed inspaced-apart relationship during circuit interrupting operation to allowfor the establishment of a circuit interrupting arc therebetwecn, eachof said electrodes having a region on which a respective electrode spotfor a low current are may be established; at least one of said electroderegions being formed of a homogeneous porous matrix having a majorproportion of tungsten and a minor proportion of thorium.

no I

2. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a pressure lower than mm. of mercury; apair of electrical terminals adapted for connection in an alternatingcurrent circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit interrupting operation to allow for theestablishment or a circuit interrupting arc therebetween, each of saidelectrodes having a region on which a respective electrode spot for alow current are may be established; at least one of said electroderegions being formed of a homogeneous porous matrix composed ofsubstantially 7% thorium, substantially 25% binding agent, and thebalance tungsten, the matrix being impregnated with copper.

3. A vacuum alternating current circuit interrupter comprising: anevacuable envelope evacuated to a pressure lower than 10 mm. of mercury;a pair of electrical terminals adapted for connection in an alternatingcurrent circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit interrupting operation to allow for theestablishment of a circuit interrupting arc therebetween, each of saidelectrodes having a region on which a respective electrode spot for alow current are may be established; said electrode regions each beingformed of materials incapable of providing a copious supply of vaporunder the influence of the are at least one of said electrode regionshaving a 2 to weight percent of low work function material homogeneouslydistributed therein.

4. A vacuum alternating current circuit interrupter comprising: anevacuable envelope evacuated to a pressure lower than 10* of mercury; apair of electrical terminals adapted for connection in an alternatingcurrent circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit interrupting operation to allow for theestablishment of a circuit interrupting arc therebetween, each of saidelectrodes having a region on which a respective electrode spot for alow current are may be established; said electrode regions eachincluding a major proportion of refractory material and incapable ofproviding a copious supply of vapor at the instant of arc interruption,at least one of said electrode regions having 2 to 15 weight percent ofa material selected from the class consisting of thorium, caesium,lanthanum, rubidium and barium.

5. A vacuum alternating current circuit interrupter comprising: anevacuable envelope evacuated to a pressure lower than 10 mm. of mercury;a pair of electrical terminals adapted for connection in an alternatingcircuit; a pair of electrodes located within said envelope, connected incircuit between said terminals and disposed in spaced-apart relationshipduring circuit-interrupting operation to allow for the establishment ofa circuitinterrupting arc therebetween, each of said electrodes having aregion on which a respective electrode spot for a low current are may beestablished; at least one of said electrode regions being formed of ahomogenous coarse matrix having a major portion of a refractory metalselect-ed from the group consisting of tungsten, tantalum andmolybdenum, and a minor portion of a low work function material selectedfrom a group consisting of thorium, caesium, lanthanum, rubidium andbarium.

6. A vacuum alternating current circuit interrupter comprising: anevacuablc envelope evacuated to a pressure lower than 10* mm. ofmercury; a pair of electrical terminals adapted for connection in analternating circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit-interrupting operation. to allow for theestablishment of a circuitinterrupting arc thercbetween, each of saidelectrodes having a region on which a respective electrode spot for alow current are may be established; at least one of said electroderegions being formed of a homogenous coarse matrix composed ofsubstantially 2 to 15 weight percent of thorium, enough binding agent toallow for the formation of a sintered porous matrix and the balancebeing tungsten, the matrix being impregnated with copper.

7. A vacuum alternating current circuit interrupter comprising: anevaculable envelope evacuated to a pressure lower than 10* mm, ofmercury; a pair of electrical terminals adapted for connection in analternating circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit-interrupting operation to allow for theestablishment of a circuitinterrupting are thcrebetween, each of saidelectrodes having a region on which a respective electrode spot for alow current are may be established; said electrode region each includinga refractory matrix having a major proportion of refractory materialwhich is incapabio of providing a copious supply of vapor at the instantof are interruption, at least one of said electrode matrices containing2 to 15 Weight percent of a material selected trom the class consistingof thorium, caesium, lanthanum, rubidium and barium.

References Cited in the tile of this patent UNITED STATES PATENTS

